In an image reading device such as a scanner used in an image forming apparatus or MFP (Multi Functional Printing) system, a signal output from a CCD (Charge-Coupled Device) upon photoelectric conversion must be amplified by using an amplifier before image processing. This is because outputs from the CCD contain variations in the light amount of the lamp, changes in light amount due to secular changes in the lamp, variations in mirror reflectivity, variations in CCD sensitivity, and the like, and hence need to be set at a predetermined level before image processing.
Conventionally, a gain is set in such a preamplifier in the following manner.
First of all, the gain is set in advance to the minimum magnification in the early stage. The exposure lamp is then turned on to read light reflected by a white reference plate by using the CCD. The analog value of an output signal from the CCD is converted into a digital value and compared with a target value.
If they do not coincide with each other, the set gain value of the preamplifier is increased from the minimum magnification by a predetermined value. As described above, light reflected by the white reference plate is read again by the CCD. The analog value of an output signal from the CCD is converted into digital value and compared with the target value.
If they do not coincide with each other, the set magnification is increased by a predetermined value. When a read output value reaches the target value or more after repetition of such operation, the gain setting operation is finished. The gain is set to the resultant value.
Such a conventional gain setting method takes much time to obtain a gain with which an output signal having a target value can be obtained from the CCD. This prolongs the time between turning on the power of the image reading device and starting read operation. As a consequence, in an image forming apparatus or MFP system, the first copy time slows down, resulting in low customer satisfaction level.
The present invention has been made in consideration of the above situation, and has as its object to provide an image reading device and a gain setting method in the image reading device, which can shorten the time taken for the image reading device to start read operation.